The invention relates to an integrated combination of thermionic energy conversion and thermoelectric energy conversion in a single device. The integrated thermionic/thermoelectric physical process and device are designated by the name “TITE.”
A thermionic energy converter is a gaseous electronic device by which heat is converted directly into electric power non-mechanically by thermionic emission or evaporation of electrons from a hot electrode or emitter and their collection or condensation in a colder electrode or collector. A thermoelectric energy converter, or thermocouple, is a solid state device by which heat is converted into electric power by diffusion of electrons and holes down a temperature gradient, and up a potential gradient, in series-connected solid electrical conductors, typically n and p type semiconductors. Typically, the thermoelectric converter comprises an n-type leg paired in electrical series with a p-type leg.
The physical principles and technology of both types of energy conversion are well-understood and have been reduced to engineering practice in practical devices. However, both types of devices have limitations which reduce performance. Certain aspects of earlier devices and background can be found in E. H. Rhoderick, “Metal-semiconductor Contacts”, Clarendon Press, Oxford, 2nd edition, pages 202-204, 1988; N. S. Rasor, “Thermionic Energy Conversion Plasmas”, invited review, IEEE Trans. on Plasma Sci. Vol. 19, pages 1191-1208 (1991).
In thermionic energy converters the negative space charge of the electron gas, unless compensated, produces an electron potential energy barrier which limits the electron current during its passage between the hot and cold electrodes. This space charge and the resulting barrier can be suppressed by an electron-accelerating electric field between the electrodes or by introduction of positive ions to form a neutral plasma, allowing a much larger interelectrode spacing than would be possible using a vacuum.
Another basic performance limitation in thermionic energy converters is excessive electron energy loss at the collector. The electron gas reaching the collector is much hotter than the collector and its available thermal kinetic energy, 0.2-0.3 eV, (eV=electron volts), accordingly is wasted upon collection. Also, the electrons lose even more potential energy as they are collected across the potential energy barrier at the collector. This electron potential energy barrier at a surface. which is essentially the heat of electron vaporization or condensation, is known as the work function. Because the lowest work function practically available is greater than φC˜1.4 eV, the output voltage of the converter is greatly reduced and low temperature heat rejection cannot be effectively used. Thus, the emitter temperature must be very high to obtain practical output currents. In general, the efficiency of current thermionic devices that utilize plasma is in the range of 25-30% of Carnot efficiency.
The electrical and thermal performance of thermoelectric energy conversion also is limited primarily by two physical processes. First, the hot electron gas must diffuse up a potential gradient through the crystal lattice of a solid from the device's hot side to its cold side. This means that the forward current of electrons is reduced by a nearly equal counter-current that greatly limits the magnitude of the net output current obtainable at a given output potential difference. It also means that about half of the electrical power generated is dissipated by ohmic resistance to electron flow through the solid.
The second primary limitation in thermoelectric energy conversion is that a continuous solid path for heat conduction exists between the heat source and the heat sink. Since non-productive heat conducted through the solid generally is much greater than the heat transported by the electron current, the basic thermal efficiency of thermoelectric conversion is small, only about 10-15% of Carnot efficiency. It is difficult to find materials that are both appropriate for thermoelectric energy conversion and have sufficiently low thermal conductivity, since reducing lattice phonon heat conduction requires weak inter-atomic bonds, and weak bonds are accompanied by thermal and mechanical instability of a solid.
It is an objective of the current invention to provide a device that allows conversion of heat into electrical energy that is more efficient than current thermionic and thermoelectric devices.
It is a further objective of the invention to provide a device that will function effectively at a broader range of heat source and heat sink temperatures than existing thermionic and thermoelectric devices.